1. Field of the Invention
The present invention relates to a solid state television camera which uses a semiconductor element, for example, a charge coupled device, as an image sensor, and more particularly is directed to a solid state television camera, as aforesaid, which has an improved circuit for eliminating noise caused by defects in the semiconductor material or crystal of the charge coupled device.
2. Description of the Prior Art
Solid state television cameras have been proposed in which charge coupled devices (hereinafter referred to as CCDs) are used as solid state image sensors. A CCD is constructed by forming a plurality of electrodes with a predetermined pitch therebetween on an insulating layer covering one surface of a semiconductor crystal substrate. The optical image of an object is projected on the image sensor from the side having the electrodes thereon or from the other side of the substrate. The projected optical image generates a corresponding pattern of charge carriers in the substrate under the electrodes. These charges are then read out successively from the CCD or transferred to a storage array of CCD elements by clock pulses applied to the electrodes.
Since it is rather difficult to make the semiconductor crystal of a CCD uniform over a substantial area, local crystal defects often are produced, and electric charges are apt to be thermally produced at such defects with a resultantly abnormally large dark current. As a result, when the camera using the above described CCD picks up an object whose image is projected on the CCD, noise appears in the resulting output signal at those portions where the dark current is abnormally large. Such noise having a higher level than the white level is mixed in the output signal of the camera and undesirably appears in the reproduced picture.
In the prior art, in order to remove such noise, it is proposed to use a memory circuit in which the positions of crystal defects in the semiconductor substrate are memorized. The output signal of the CCD is controlled with the output of the memory circuit to eliminate the noise, for example, by passing the output of the CCD through a sample-and-hold circuit which is made to sample the CCD output signal only so long as the memory output indicates that the respective picture element of the CCD is free of any crystal defect.
Normally, the existence or non-existence of a crystal defect is memorized for every picture element of the CCD. For a CCD having a matrix of picture elements with N.sub.H picture elements in the horizontal direction and N.sub.V picture elements in the vertical direction, the memory circuit requires a memory capacity of N.sub.H . N.sub.V bits. In the usual reproduced picture of a television receiver, N.sub.H is 300 to 500, and N.sub.V is 200 to 300. Accordingly, if the positions of the crystal defects are memorized in respect to all of the picture elements, a memory with a very high capacity, for example, from 60,000 to 150,000 bits, is required. Such a memory circuit is very expensive and, therefore, it is difficult to provide an inexpensive solid state television camera of the described kind.
In order to reduce the required memory capacity, the existence or non-existence of a crystal defect is not memorized for every picture element in order, but rather only the positions of those picture elements having the crystal defects may be encoded and memorized in the memory circuit. For example, the positions of the picture elements having crystal defects are encoded with respect to the X-axis and Y-axis of the orthogonal coordinates on the semiconductor substrate. When the number N.sub.H of the picture elements in the horizontal scanning direction is about 500, the position of any one of the picture elements in the horizontal scanning direction can be expressed with the capacity of 9 bits. Similarly, when the number N.sub.V of the picture elements in the vertical scanning direction is about 250, the position of any one of the picture elements in the vertical scanning direction can be expressed with the capacity of 8 bits. When an interlace scanning method is employed, one additional bit is required to identify whether the field is even or odd. Therefore, a total of 18 bits are required to express the position of each of the picture elements having a crystal defect with respect to the X-axis and Y-axis of the orthogonal coordinates and to provide identification of the field. If the maximum permissible number of picture elements with crystal defects is 20 for a usable CCD, the memory may have a capacity of only about 400 bits. A memory with the foregoing low capacity can be economically provided. However, as will be hereinafter indicated, the known noise eliminating circuit requires relatively complicated circuit elements, such as, address counters and a coincidence circuit, in association with the memory.